This invention relates to a method for purifying sulfuric acid solutions of iron to render the solutions suitable for use in ion membrane processing.
In the manufacture of tungsten a number of by-products are produced which have to be either disposed of or used as an alternate product.
In the processing of tungsten ores, the ore is normally digested in strong base, usually sodium hydroxide, to produce sodium tungstate solution. This solution is purified to remove silicon, phosphorus, arsenic, molybdenun and other impurities. The purified solution is acidified with sulfuric acid to produce a solution of sodium metatungstate and other tungsten species. This solution is feed solution for solvent extraction processing. The tungsten is extracted by an organic extracting agent. The tungsten is stripped from the organic normally by ammonia. The stripped organic is regenerated with sulfuric acid and can then be reused in the solvent extraction process. The solution from which the tungsten is extracted, the raffinate, is a solution of sodium sulfate. This solution can be processed by ion membrane technology to recover sodium hydroxide and sulfuric acid which are usable products. This process avoids production of a waste stream. In ion membrane technology, a cell using ion exchange membranes is used. This cell is a critical part of the process and requires the use of purified solutions since any ion above monovalent contaminates the membranes and renders them inoperative. The sulfuric acid which is used to regenerate the organic becomes contaminated on contact with the organic with small amounts of iron, tungsten, calcium, magnesium and organic. The sulfuric acid concentration in this spent solution is also too low for reuse, about 5% by volume. However, this acid can be added to make up water in the ion membrane process and can therefore be concentrated. Therefore the acid can be reused, thereby avoiding formation of a waste stream. However, in order to be used in the ion membrane process, the sulfuric acid must be purified.
Therefore a process to purify such sulfuric acid solutions would be desirable.